Internal combustion engines are ideally operated in a way that the combustion mixture contains air and fuel in the exact relative proportions required for a stoichiometric combustion reaction (i.e., where the fuel is burned completely.) A rich-burn engine may operate with a stoichiometric amount of fuel or a slight excess of fuel, while a lean-burn engine operates with an excess of oxygen (O2) compared to the amount required for stoichiometric combustion. The operation of an internal combustion engine in lean mode may reduce throttling losses and may take advantage of higher compression ratios, thereby providing improvements in performance and efficiency. Rich burn engines may have the benefits of being relatively simple, reliable, stable, and adapting well to changing loads. Rich burn engines may also have lower nitrogen oxide emissions, but at the expense of increased emissions of other compounds.
In order to comply with emissions standards, many rich burn internal combustion engines utilize catalysts, such as non-selective catalytic reduction (NSCR) subsystems (commonly known as three-way catalysts). Catalysts may reduce emissions of the nitrogen oxides NO and NO2 (collectively NOx), carbon monoxide (CO), ammonia (NH3), methane (CH4), other volatile organic compounds (VOC), and other compounds and emissions components by converting such emissions components to less toxic substances. This conversion is performed in a catalyst component using catalyzed chemical reactions. Catalysts can have high reduction efficiencies and can provide an economical means of meeting emissions standards (often expressed in terms of grams of emissions per brake horsepower hour (g/bhp-hr)). Separate catalyst components or devices may be included in the exhaust pathway of a rich burn engine to convert different emissions components. For example, one catalyst component may convert carbon monoxide and NOx while another may convert ammonia and methane.
In the oxidation process, the resulting substances generated by a catalyst component may require further conversion by a subsequent catalyst. For example, a catalyst component may convert NOx generated by an engine into ammonia, which may then be converted by another catalyst component. In a rich burn engine, controlling carbon monoxide and NOx emissions poses many challenges, one of which is operating the engine within an operating window of air/fuel proportions that allows the catalyst components to perform optimally, reducing emissions to the maximum extent possible. The air/fuel proportion window for optimal performance of natural gas engines is relatively narrow, thus hindering the ability to operate the engine at a richer burn that would reduce NOx emissions. Moreover, operating such an engine within a desired operating window can require many components and regular calibrations, thus making such an engine costly to maintain.